gdb/testsuite: remove incorrect global variable accesses

gdb/testsuite: remove incorrect global variable accesses

I noticed in gdb.replay/missing-thread.exp a reference to $testfile in
a location where 'testfile' had not been made available via a use of
'global'.  The uses looked like this:

  unsupported "$testfile (couldn't start gdbreplay)"

I think there are three problems here, the $testfile is invalid
because there's no 'global testfile' making the variable available.
The use of $testfile is redundant anyway as 'unsupported' already adds
the script name to the output line.  The final text within parenthesis
is bad style that's an important part of the output, but GDB test name
style is that text in parenthesis is additional text that could be
ignored, e.g. "(timeout)".

Replace the above with just:

  unsupported "couldn't start gdbreplay"

This same construct has been copied into multiple gdb.replay/ tests,
so fix them all.

There's no change to what is actually tested after this commit.